1. Field of the Invention
The present invention relates to an information recording system, and more particularly to a recording system which utilizes a recording medium having a vertical magnetization film on and from which the information is recorded, and having read out by an optical means employing such as a laser beam.
2. Description of the Prior Art
In recent years, recording systems have been proposed within the information is recorded and played back by effectively utilizing the magneto-optical charactetistics of an amorphous (noncrystalline) alloy material which is used as the recording medium.
This is due to the fact that the amorphous alloy thin film, such as GdFe (Gadlinium Iron) or GdCo (Gadlinium Cobalt), is a "vertical magnetization film" in which the direction of magnetization is perpendicular to the surface of the film.
Recording of the information on this amorphous thin film takes place in a manner such that the orientation or the direction of magnetization of the desired portions of the amorphous alloy film which is initially magnetized in a preselected direction is turned over by heating those portions above the Curie temperature or a compensation temperature under the conditions of application of a weak external magnetic field.
More specifically, the amorphous alloy thin film which is previously downwardly (corresponding to the binary "0" signal) magnetized is applied with a weak upward bias magnetic field and only the portions of the thin film on which the "1" signal is to be recorded are applied with the laser beam so as to raise the temperature of those portions above the Curie temperature or a compensation temperature to cause the change in the direction of magnetization.
On the other hand, the reading out of the information recorded on the thin film takes place by utilizing the magnetic Kerr effect which is an interaction between the magnetism of substance and the light beam.
When a linearly polarized laser beam is reflected by the magnetized thin film, the plane of polarization is rotated in a direction determined by the direction of magnetization.
Therefore, the information recorded on the amorphous thin film is read out by detecting the direction of rotation of the plane of polarization of the reflected laser beam by means of an analyzer.
These kind of recording systems, however, have suffered from the problem that a focus lens of the recording laser beam is moved up and down each time the recording is started and stopped. Specifically, these systems include a focus servo mechanism for operating the movement of the focus lens, the servo mechanism being actuated only when a focus servo error signal, obtained upon the presence of the recording laser beam which depends on the recording signal, is present.